1. Field of the Invention
The present invention relates to a steering control system, which controls a transmission ratio (hereinafter, referred to as a steering ratio) between a steering angle of a steering wheel and a steering angle of steerable wheels and which provides an assist for a steering force.
2. Description of Related Art
Previously, a variable gear ratio steering (VGRS) apparatus, which controls the steering ratio based on a traveling speed of the vehicle, has been known. At this VGRS apparatus, the steering angle of the steerable wheels relative to the steering angle of the steering wheel is increased at the low traveling speed of the vehicle to improve the maneuverability of the vehicle. In contrast, the steering angle of the steerable wheels relative to the steering angle of the steering wheel is decreased at the high traveling speed of the vehicle to improve the safety of the vehicle.
Furthermore, an electric power steering (EPS) apparatus, which controls the steering force based on the steering torque, has been known. At this EPS apparatus, the steering torque is sensed based on a twist angle of a torsion bar, which is installed to an output shaft of the VGRS apparatus, and the steering force is assisted based on the sensed steering torque. In this way, the maneuvering work load of the driver of the vehicle can be advantageously reduced.
Japanese Unexamined Patent Publication No. 2005-221053A (corresponding to U.S. 2005/0173184A1) teaches one such VGRS apparatus to control the steering ratio. In this VGRS apparatus, an input shaft is connected to a steering wheel, and an output shaft is connected to a steerable wheel side and is placed coaxial with the input shaft. Input gear, which is rotated integrally with the input shaft, and an output gear, which is rotated integrally with the output shaft, are opposed to each other. Intermediate gears are meshed with both of the input gear and the output gear and are rotated about a rotational axis of the input shaft and of the output shaft.
However, in the VGRS apparatus of Japanese Unexamined Patent Publication No. 2005-221053A (corresponding to U.S. 2005/0173184A1), each support shaft, which rotatably supports a corresponding one of the intermediate gears, is formed integrally with a rotation support member, which is placed perpendicular to the support shaft and is coaxial with the input shaft and the output shaft. Therefore, the processing of the rotation support member is difficult. Thereby, it is difficult to increase the number of the support shafts and the number of the intermediate gears to increase a torque transmission capacity between the input gear and the output gear.
Furthermore, in the VGRS apparatus of Japanese Unexamined Patent Publication No. 2005-221053A (corresponding to U.S. 2005/0173184A1), the rotation of the input gear is transmitted to the output gear through the intermediate gears. Therefore, the rotational direction of the output shaft is reversed from the rotational direction of the input shaft. Thereby, in the case where this VGRS apparatus is used in the vehicle that has the steering control system, in which the steering wheel and the output shaft rotate in the same common direction, a gear device, which reverses the steering angle of the output shaft once again, needs to be placed between the output shaft and a steering pinion gear. However, when such a gear device is provided, the size of the entire steering control system may possibly be increased.
Furthermore, in the case of the VGRS apparatus of Japanese Unexamined Patent Publication No. 2005-221053A (corresponding to U.S. 2005/0173184A1), when the EPS apparatus is placed on the axial side of the VGRS apparatus where the steerable wheels are located, the torsion bar needs to be installed to the output shaft located on the axial side of the output gear where the steerable wheels are located. Therefore, the axial distance between the VGRS apparatus and the EPS apparatus is lengthened, and the installability of the VGRS apparatus to the vehicle is deteriorated.